Welding of coated metals

ABSTRACT

A method and apparatus for welding together two metal workpieces, at least one of which has thereon a coating material which enters a fluid or visco-elastic phase at a temperature below the melting temperature of the workpieces, the method comprising the steps of subjecting the workpieces to a preliminary preparative treatment at a zone to be welded and thereafter applying (preferably be means of a high-energy beam such as a laser beam) sufficient heat to the workpieces at the weld zone to effect there the welding together thereof, wherein the preliminary preparative treatment comprises preheating the workpieces to the said lower temperature and pressing them together, at the zone to be welded with a force sufficiently great to expel coating material from between the workpieces at the weld zone. thereby to improve the quality of the subsequent weld.

This invention relates to methods of welding coated metals such as steelcoated with zinc, and to apparatus for welding such coated metals. Moreparticularly, but not exclusively, this invention relates to processesfor welding galvanized sheet steel as an integral step in themanufacture of various products, such as automobile body shells.

The use of welding in a wide variety of manufacturing processes is veryfamiliar in industry. Typical equipment for performing one such processcomprises, in addition to the workpieces to e welded, a blow torch witha supply of gas that can be regulated to provide a controllable heatsource and, for some kinds of welding, an appropriate welding flux. Thedifficulties of controlling accurately the heat source in this processtogether with the inconvenience of having to make use of a suitable fluxproduce considerable limitations on the quality and neatness of aresulting weld. For these reasons, conventional gas welding processeshave been larely superseded in many instances by laser weldingprocesses, usually employing CO₂ lasers which can serve as accuratelyfocused and well controlled heat sources. Additionally, the use of alaser in a welding process enables the use of a welding flux to bedispensed with and this leads to very firm, sound and neat welds in thefinished product. There are additional advantages in that smooth,efficient, accurate and swift processing can be achieved.

Steel sheets are often purchased as a processing material already coatedwith zinc to prevent corrosion. A basic difficulty arises when attemptsare made to weld such galvanized steel sheets, especially when a laserwelding process is to be employed.

Galvanized steel may be produced by one of the following methods:spraying a steel surface with molten zinc, electrolytic deposition ofzinc on steel, heating steel in contact with zinc dust, and dippingsteel into molten zinc. The zinc coated surface of the steel, whenexposed to air, acquires a coherent inert oxide layer which prevents thesteel from corroding. If the zinc layer on the surface of the steel ifborken, some protection against corrosion is still obtained, as zinc ismore electropositive than iron and the first stage of oxidation of zinc

    Zn→Zn.sup.++ +2e.sup.-

occurs in preference to that of iron

    Fe →Fe.sup.++ +2e.sup.-

It is for these reasons that galvanized steel is used extensively inindustry. However, industrial production processes frequently requirethe lap welding of steel sheets, and, when galvanized steel sheets areused, the welding process has to cope with the layers of zinc betweenthe steel substrates. Special difficulties arise with such sheets whenwelding by the use of a high-energy beam such as an electron beam or alaser beam. This is because zinc boils at a temperature of about 907°C., but steels melt at somewhat higher temperatures of about 1372° C.,leading to explosive vaporization of the zinc layers sandwiched betweenthe steel substrates of the sheets and consequent irregular distributionof (and loss of material from) the weld puddle of molten steel producedby the high-energy beam where it impinges on the sheets. The resultingweld is, in consequence, likely to suffer from voids and porosity and tobe of poor quality.

One possibility for overcoming this problem is to machine the zinccoating off the steel sheet. This suffers from the disadvantages ofbeing costly, time consuming and labour-intensive. Also there could beregions where the zinc coating is imperfectly removed by machining;inevitably problems would then be produced.

Another possibility is to follow the procedures suggested by the FordMotor company in their U.S. Pat. No. 3,969,604. This describes a methodof welding glavanized steel, using a high-energy welding beam, whichattempts to prevent rapid vaporization of the zinc coating so as toavoid disrupting the weld puddle. In connection with the use of electronor laser beam welding with a power density of 6200 W/mm², the patentproposed painting the galvanized steel workpieces, in the area adjacentthe weld zone, with a coating of a flux material such as iron oxidewhich, on heating, forms with the zinc coating a compound having avapour pressure equal to or lower than that of the steel substrate. Itis claimed that explosive vaporisation of the zinc and consequentdisruption of the weld puddle are thus prevented, and that the problemof voids or porosity of the weld is thereby overcome.

This suggested process is, however, subject to drawbacks. One is thatthe use of an additional flux material has the effect of making the finecontrol of the process difficult, under industrial manufacturingconditions. By contrast, a purely physical procedure without essentiallychemical involvement is more likely to be suitable for a manufacturingprocess.

It is a principal object of the present invention to provide a method ofwelding coated metals, such as zinc-coated (galvanized) steel sheets,and apparatus for welding such metals, which overcome the disadvantagesas described above for the known procedures.

Accordingly, one aspect of the present invention provides a method ofwelding together two metal workpieces, at least one of which has thereona coating material which enters a fluid or visco-elastic phase at alower temperature than the melting temperature of the workpieces, themethod comprising the steps of subjecting the workpieces to apreliminary preparative treatment at a zone to be welded and thereafterapplying sufficient heat to the workpieces at the weld zone to effectthere the welding together thereof, wherein the preliminary preparativetreatment comprises preheating the workpieces to the said lowertemperature and pressing them together, at the zone to be welded, with aforce sufficiently great to expel coating material from between theworkpieces at the weld zone.

In most instances, probably, the preheating will be carried to atemperature at which the coating material in the weld zone liquifies;but there are some potentially useful coating materials, such as somealloys of zinc, in which preheating to the fully molten liquid state isunnecessary because that state is preceded at a lower temperature by anacceptable "visco-elastic" state in which the material assumes aconsistency resembling that of butter or putty which allows it to beexpelled by the application of pressure from between two coatedworkpieces.

According to a further aspect of the invention there is provided weldingapparatus for carrying out the method of the invention, such apparatuscomprising pretreatment means, welding means, and means facilitating thetransport of workpieces to be welded from the pretreatment means to thewelding means, wherein the welding means comprises means for heating theworkpieces to a welding temperature and the pretreatment means comprisesmeans adapted to heat the workpieces to a lower temperature and meansfor pressing the workpieces together.

The preheating step in the method provided by the invention may beeffected in various ways. Rollers or other means by which pressure isapplied to the workpieces to expel coating material from between themmay be made to serve as electrodes for passing a resistive-heatingelectric current through the workpieces to effect the preheating, or ahigh-energy beam such as a laser beam may be used, both for thepreheating step and for the heating by which the actual welding issubsequently effected. In particular, if the further heating by means ofwhich welding is effected is applied by means of a high-energy laserbeam, the beam may be split into major and minor parts, with the minorpart being diverted to effect the preheating and the major part, ofgreater power, being used to effect the weld at the weld zone.

It is within the scope of the invention to allow sufficient cooling ofthe workpieces to occur, between the preliminary pretreatment step andthe actual welding step, that any unexpelled residue of the coatingmaterial within the zone to be welded, as well as adjacent unexpelledcoating material which has been liquified or softened by the preliminarytreatment, may return to its solid state and may, indeed, have theeffect of soldering the workpieces together prior to being reheatedduring the actual welding step.

It will be understood that the welding process according to theinvention lends itself to operation on a continuous basis, though it mayalso be performed on individual workpieces.

It will be appreciated that the welding process of this inventionparticularly lends itself to the lap welding of galvanised steel sheets.In such a case, the preheating should raise the sheet temperature toabout the melting point of zinc, to ensure substantially all the zinc issqueezed out of the overlapping sheet regions to be welded together.

A laser, such as CO₂, laser may be considered a preferred means ofproviding the heating in the actual welding step, for such a heat sourcegives accurate, clean and controlled heating to effect welding of theworkpieces. The apparatus most preferably includes pressure means,adapted to urge the two workpieces into engagement with one anotherimmediately after the workpieces have been heated to the weldingtemperature, to ensure that a sound and homogenous weld is completed.

It will be appreciated that in performing the process of this invention,no additional flux material need be employed. The act of squeezing outthe coating material (such as the zinc in the case of galvanised steelsheets) from the weld zone protects that weld zone during the subsequentwelding step. In addition to lap seams, continuous butt welds may beformed by the process.

In order that the invention may better be understood, it will now bedescribed in greater detail, reference being made to the accompanyingdrawings, in which:

FIG. 1 is a diagrammatic view of a continuous lap welding apparatus ofthis invention;

FIG. 2 is a similar view of a lap welding apparatus similar to thatshown in FIG. 1 but incorporating some modification;

FIG. 3 is a plot of applied force against the percentage of zincexpected to be removed, when welding galvanized steel sheets by themethod of the invention;

FIG. 4 shows corresponding plots relating to the results of certainexperimental results obtained in relation to the method of thisinvention;

FIGS. 5 and 6 are photographs of sample welds; and

FIG. 7 is a diagrammatic cross-section through a weld completed inaccordance with this invention.

The embodiment of apparatus of this invention for performing a weldingprocess also of this invention and illustrated in FIG. 1 is intended toeffect continuous lap welding on two zinc-coated (galvanized) steelworkpieces. The apparatus includes a base 11 supporting a worktable 12and upstanding arms 13 and 14 in which are journalled shafts 15 and 16of two worktable rollers 17 and 18 which extend upwardly throughrespective slots 19 and 20 in the worktable so as to project slightlyproud of its upper surface. The arms 13 are formed with slots 21 inwhich are vertically slidably mounted journal blocks 22 which aredownwardly spring-urged by springs 23 and have journalled in them ashaft 24 of a pressure roller 25 which is thus spring-urged towards theroller 17 to define a pressure nip between those two rollers. A furtherpressure roller 26, similarly spring-urged towards the roller 18 anddefining therewith a further pressure nip, is carried on a shaft 27journalled in blocks 28 slidably mounted in slots 29 of the arms 14 andurged downwardly by springs 30. A suitable current source (not shown) isconnected to the rollers 17 and 25 as indicated schematically by brushes31 and 32, whereby an electric current may be passed through thesuperimposed workpieces, 33 and 34, as they are advanced togetherthrough the nip between rollers 17 and 25, in the direction of arrow A.A CO₂ laser 35 is positioned to direct a focused beam 36 on to a weldzone 37 of the workpieces as they progress between the rollers 17 and18.

The apparatus is operated by first ensuring that the workpieces 33 and34 travel smoothly over the guiding worktable 12 and worktable rollers17 and 18 and through the pressure roller nips in a continuous way, at asuitable horizontal speed. The pressure of the upper roller 25 isadjusted, by means not shown, to a sufficiently high value. Anappropriate current is selected, having regard to the horizontal travelspeed of the workpieces, for passing through the workpieces at the nipbetween the rollers 17 and 25, to ensure a preheating of the workpiecessufficient to melt their zinc coatings. The welding laser 19 ismeanwhile adjusted to be capable of performing a continuous weldingprocess on the workpieces 33 and 34 at the weld zone 37.

On passing between the pair of rollers 17 and 25, the zinc on theworkpieces is heated and squeezed out of that area, due to the pressureexerted by the rollers, to be reduced to a level where any remainingzinc will no longer present a problem in the laser welding process. Thelaser welding then proceeds in the usual manner, at the weld zone 37,and the welded workpieces are guided away through the nip between therollers 18 and 26 which apply pressure to hold the sheets in theircorrect relationship as the weld between them solidifies.

It will be understood that the means by which the preheating is achievedis not a critical feature of the invention. For example, instead ofusing electrical resistive heating as described above with reference toFIG. 1, there could be provided in advance of the rollers 17 and 25, asshown in broken line, a further laser 38 of lower power than the laser35 but sufficient, when incident on the sheet 34, to provide therequired degree of pre-heating to melt the zinc coatings locally betweenthe two sheets 33 and 34. In that case the rollers 17 and 25 do notprovide the preheating, but only the pressure by which the molten zinc,along a line traced with respect to the sheets 33 and 34 as they passunder the laser 38, is squeezed out from between the sheets as therollers press them together and slightly deform them towards and intocontact with one another along that line.

Instead of providing a separate laser 38 for effecting the preheating, apractical alternative may be to split the output of the laser 35 bymeans not shown into a major beam which then serves as the welding beam36 and a minor beam which is directed on to the sheet 34 ahead of therollers 17 and 25 to effect the preheating.

It will be appreciated that the process described above in variousmodifications allows the rapid welding of galvanized steel sheets in acontinuous manner. The preliminary pretreatment in accordance with theinvention compares favourably with a mechanical machining process toremove zinc, which would be difficult to carry out continuously and tocombine with a continuous welding operation, and which in any eventwould be time-consuming, liable to be erratic, labour-intensive and thusexpensive. By contrast, the pretreatment step according to the inventionis entirely based on physical processes which can be easily carried outin a smooth, controlled and accurate manner, and without the need foraddition of any flux material. The process according to the invention isthus rapid, smooth, efficient and economical, compared to knownprocesses for welding such sheets.

A further modification of apparatus for welding in accordance with theinvention is illustrated in FIG. 2, in which the pair of rollers 17 and25 is omitted and there are provided instead a fixed elongate electrode39 which projects slightly through the slot 19 of the worktable 12 forsliding contact with the sheet 33 and a movable elongate electrode 40which is reciprocable vertically, by means not shown but as indicated bythe double-headed arrow B, into and out of a lower position in which thesheets 33 and 34 to be welded are clamped between it and the fixedelectrode 39. Essentially the electrodes 39 and 40 resemble those of aspot welding machine and are controlled in a similar manner: while theyclamp the sheets 33 and 34 between them a heating current source isconnected to them to pass a heating current through the sheets 33 and34. In this context, however, the heating current is chosen to besufficient to melt the zinc coating of the sheets, locally between theelectrodes, but insufficient to melt the steel of the sheets and thusweld them together. The sheets 33 and 34 in this instance are advancedstep-wise, with alternating stationary periods and periods of workpieceadvance. During each stationary period the sheets are gripped betweenthe electrodes 39 and 40, the preheating current passes to melt the zinccoatings between the sheets and the molten zinc is expelled locally frombetween the sheets by the pressure exerted by the electrodes. At the endof each such stationary period, the preheating current is switched off,the electrode 40 is raised to release the sheets 33 and 34, and theseare then advanced, in the direction of the arrow A, under the laser 35to effect welding of the sheets.

Various weld patterns are possible. If the sheets 33 and 34 are advancedsteadily under laser 35 while its beam 37 is switched on, a continuousweld of length up to that of the electrodes 39 and 40 may be made, andif the successive welds made in successive advance periods abut oneanother an effectively continuous weld of indefinite length is obtained.On the other hand. if the laser beam 36 is allowed to impinge on theworkpieces during only a small part of each advance of the workpieces,or while they are stationary, then a pattern of spaced-apart spot-weldswill result.

The process of the present invention will now be considered in greaterdetail. As disclosed above in relation to galvanised steel sheets, theprocess conduces to a satisfactory weld by preheating the region aheadof the weld zone to raise it to a temperature high enough to melt thezinc sandwiched between the layers of steel in a lap weld. whereby theheated molten zinc layer may be squeezed out of the region to be welded.The success of the method depends on the ability of the roller nip toimpose sufficient pressure to reduce the amount of zinc trapped betweenthe layers of steel to a level where any remaining zinc will notinterfere with the welding process or the quality of the resulting weld.It may prove to be an advantage to use as large a keyhole radius aspossible in the ensuing laser welding step, to ensure that any residualtraces of zinc vapour are ventilated away, thus minimising difficultiesin the weld process.

The degree to which zinc trapped between the two layers of steel can beexpelled when molten by applying pressure depends on the thickness ofthe zinc layer, the degree of overlap of the sheets and the speed oftraverse of the sheets. For a lap weld of 1 cm width and a traversespeed of 1 cm sec⁻¹, it may be calculated (assuming a preheatingtemperature high enough to melt the zinc and a trapped zinc thickness of0.01 cm) that expulsion of 99% of the trapped zinc requires apressure-roller force of the order of 10⁶ dynes whereas to remove 90% ofthe zinc requires only, approximately, 3×10⁴ dynes. The calculatedrelationships between applied force F in dynes (up to 5×10³ dynes) andthe percentage of zinc expelled are shown in FIG. 3 (for a lap weldoverlap of 1 cm) by curves I to VI for traverse speed U through therollers and thickness H of the sandwiched zinc. as follows:

    ______________________________________                                        Curve         U (cm sec.sup.-1)                                                                        H (cm)                                               ______________________________________                                        I             1.0        0.02                                                 II            1.0        0.01                                                 III           1.0         0.005                                               IV            0.5        0.02                                                 V             0.5        0.01                                                 VI            0.5         0.005                                               ______________________________________                                    

FIG. 4 shows corresponding calculated curves, for a traverse speed Uequal to 1 cm sec⁻¹, in respect of lap weld overlaps of 3 mm and 5 mmrespectively, and the hatched area between the two curves thereforerepresents the field in which experimental results would be expected tofall for a nominal lap weld overlap of 4 mm with a tolerance of ±1 mm.The horizontal bar plots of FIG. 4 represent the ranges ofexperimentally determined percentage expulsions of the zinc at variousapplied pressure forces when carrying out the preliminary preparativepre-heating and squeezing step according to the invention on numeroussamples with such a lap weld width, of 4 mm ±1 mm. It will be seen that,although the correspondence is not exact, the practical results resemblethe theoretical in at least a qualitative way.

FIG. 5 and 6 are photographic views of two welds of galvanised 1mm-thick steel sheets, the weld of FIG. 5 having been made, inaccordance with the invention, with heat and pressure pretreatment toexpel sandwiched zinc from the weld zone as shown diagrammatically inthe sketch sectional view of FIG. 7 (to which further reference is madebelow) and the weld of FIG. 6 having been made without pretreatment toremove zinc before making the weld. As shown in FIG. 7, upper and lowersheets 34 and 33 of galvanized steel have been laser-welded by exposingthe upper sheet 34 to a laser beam of sufficient energy to melt theupper sheet locally through its whole thickness and to melt the lowersheet at least part-way through, to form a trough 41 filled with amolten-metal weld puddle which then solidifies on cooling to form a weld42. The desired smooth and satisfactory formation of the weld 42 dependson the molten weld puddle having been allowed to form and then coolwithout disruption by explosive vaporisation of trapped zinc; and thisis achieved, according to the invention. by the sheets 33 and 34 havingbeen preheated and pressed together along their zone to be welded sothat the surface coating of zinc 43 sandwiched between them at theiroverlap is made molten and squeezed out from the weld zone to provide asubstantially zinc-free contact area 44 of the sheets at the weld zone.The typical resulting smooth weld 42 is shown in FIG. 5, which is a viewof that region of the top sheet 34 in which the weld is formed and whichin FIG. 7 extends between the arrows X and Y.

FIG. 6, on the other hand, shows a weld made without first carrying outthe preliminary preparative treatment in accordance with the invention,so that the explosive vaporisation of unremoved sandwiched zinc, when itis abruptly subjected to severe heating by the welding laser beam as thebeam penetrates to the interface between the two sheets, results in lossof weld material by splattering and, generally, in disruption of theweld puddle and prevention of the formation of a smooth weld. Thisresults in the formation of a weld which is marred by porosity and bythe presence of large scale voids which are seen as dark patches in FIG.6.

We claim:
 1. A method of welding together two overlapping metalworkpieces, at least one of which has thereon a coating material whichenters a fluid or visco-elastic phase at a temperature below the meltingtemperature of the workpieces, the method comprising the steps ofsubjecting the workpieces to a preliminary preparative treatment at azone of their overlap to be welded and thereafter applying sufficientheat to the workpieces at the weld zone to effect there the weldingtogether thereof, wherein the preliminary preparative treatmentcomprises preheating the workpieces at their overlap to the said lowertemperature and pressing them together, at the zone to be welded, with aforce sufficiently great to expel coating material from between theworkpieces at the weld zone.
 2. A method of welding as claimed in claim1, wherein the preheating of the workpieces is carried to a temperatureat which the coating material liquefies in the weld zone.
 3. A method ofwelding as claimed in claim 1, wherein the heat applied to weld theworkpieces is applied by subjecting at least one of the workpieces to ahigh-energy beam incident upon it at the said weld zone.
 4. A method ofwelding as claimed in claim 3, wherein the said high-energy beam is alaser beam which causes localised fusion of the workpieces at the weldzone.
 5. A method of welding as claimed in any of claims 1 to 4 whereinthe preheating is effected by passing an electric preheating currentthrough the workpieces between a pair of opposed electrodes and pressingtogether of the workpieces is effected by means of the electrodes.
 6. Amethod of welding as claimed in claim 5, wherein the preheating iseffected by passing the workpieces into the nip between a pair ofpressure rollers and utilising the rollers as the said electrodesbetween which the said preheating current is passed through theworkpieces.
 7. A method of welding as claimed in any of claims 1 to 4,wherein the preheating is effected by directing the output of apreheating laser on to at least one of the workpieces at the said weldzone thereof.
 8. A method of welding as claimed in claim 7, wherein theworkpieces are pressed together, to expel coating material from betweenthem, after the preheating has been effected.
 9. A method of welding asclaimed in claim 1, wherein a continuous weld is produced by carryingout the preliminary preparative treatment continuously and progressivelyover the workpieces to establish an elongate pretreated zone thereof,and applying the subsequent welding step progressively along that zone.10. A method of welding as claimed in claim 1, wherein the preliminarypreparative treatment is applied in a stepwise manner to provide aseries of pretreated zones to be welded and the welding step is appliedsubsequently to each zone in sequence.
 11. Welding apparatus weldingtogether two metal workpieces, at least one of which has thereon acoating material which enters a fluid or visco-elastic phase at atemperature below the melting temperature of the workpieces, saidapparatus comprising pretreatment means, welding means, and meansfacilitating the transport of workpieces to be welded from thepretreatment means to the welding means, wherein the welding meanscomprises means for heating the workpieces to a welding temperature andthe pretreatment means comprises means adapted to heat the workpieces toa lower temperature and means for pressing the workpieces together. 12.Welding apparatus as claimed in claim 11, wherein the welding meanscomprises a laser arranged to direct a laser beam on to a workpiece tobe welded.
 13. Welding apparatus as claimed in claim 10 or claim 11,wherein the pretreatment means comprises a pair of rollers adapted toreceive the workpieces in their nip.
 14. Welding apparatus as claimed inclaim 13 comprising an electrical current source connectable to therollers to apply a resistive heating current via the rollers to theworkpieces.
 15. Welding apparatus as claimed in claim 10 or claim 11,wherein the pretreatment means comprises an electrical current sourceand, connectable thereto, a pair of electrodes movable relative to oneanother to grip the workpieces between them and to release them.
 16. Awelded product made by the method claimed in any of claims 1 to 4.